This invention relates generally to improvements in hydraulic actuators and more specifically in stiffness enhancement and load holding capability for electrohydrostatic actuators for aircraft.
Hydraulic actuators (also called rams, jacks or cylinders) position loads in response to directed hydraulic fluid from servomotors. Conventional aircraft hydraulic systems utilize one or more hydraulic pumps to supply pressurized fluid to multiple actuators. Load positioning of each specific actuator is accomplished by metering fluid flow with servovalves to one side or the other of the actuator piston. After the load has been positioned, the servovalve is closed, thus allowing the actuator to hold its position. State-of-the-art design and fabrication of servovalves has resulted in acceptable positioning and load holding stiffness for aircraft control actuators.
Conventional hydraulic actuators suffer from undersirable characteristics, which include unwanted heat generation, and the need for multiple hydraulic systems per aircraft. Unwanted heat generation occurs frequently due to hydraulic system pump pressures being set to accommodate the highest load that any one actuator on an aircraft may encounter. Since most actuator operation is on an intermittent basis and at low loading conditions, much energy is wasted in providing high pressure hydraulic fluid in a standby mode. Servovalve quiescent leakage also contributes to unwanted heat generation, due to manufacturing tolerance limitations. The overall heat generated is transferred through the hydraulic system and ultimately reduces the life of actuator components. Cooling systems (heat exchangers) are required to transfer the heat away from the hydraulic fluid, leading to system weight penalities. Aircraft safety requires that redundant piping systems be used for all flight critical control surfaces. This adds weight, complexity, and reduced hydraulic system reliability.
In moving to make modern aircraft more fuel efficient, safe, and less vulnerable, government agencies have requested the development of actuation systems which do not require the need for centralized hydraulic systems. The Integrated Actuator Package (IAP) and Electrohydrostatic Actuator (EHA) are two potential solutions to this problem.
The IAP is an actuator which is powered by an actuator mounted electric motor driven hydraulic pump. The electric motor is of constant speed and of unidirectional rotation. Actuator direction is controlled by a servovalve which directs fluid flow in a conventional manner, or by controlling an over-center variable displacement pump mechanism. The latter method controls both hydraulic flow direction and flow rate. The IAP eliminates the piping of the conventional hydraulic system, but not the waste heat generated by a conventional system. This is because the motor and pump must rotate at a constant speed to maintain a high fluid pressure.
The EHA is similar in design to the IAP, in that an electric motor driven pump is mounted to the actuator. The major difference between the two configurations is that the EHA electronic controller actually reverses the electric motor and pump direction to change actuator direction, thus eliminating the need for a servomotor or other position control mechanisms. The EHA electronic controller generates the speed and direction of rotation for the motor which in turn develops the pressure and flow required to position and hold the actuator load. The EHA electronic controller is more complex than the IAP controller, however fluid heating is significantly reduced and external cooling is not required. The end result is an EHA that operaters in a true power on demand mode which is energy efficient.
The elimination of the conventional servovalves in EHAs creates deficiencies in which uncommanded static and dynamic load carrying capability may be lost. The capability to hold static and dynamic loads in the presense of load disturbances is referred to as actuator stiffness. The loss of stiffness is primarily linked to the EHA's inability to seal off the actuator cylinder ports at both static and oscillating load conditions. Fluid compressibility is also an important contributor, since it is affected by fluid type, temperature, air content, and operating pressure. Other factors which can affect stiffness are piston area, cylinder volume, and component mechanical stiffness.
Conventional actuators use a high operating pressure to counteract the degradation in fluid stiffness caused by high operating temperatures and dissolved and entrained air. EHAs and IAPs have closed fluid systems, therefore they can be completely deaerated. The EHA can also be operated at lower fluid temperatures since it is a power on demand system, resulting in a fluid stiffness that is greater than or equivalent to systems utilizing high operating pressures.
Conventional actuators are typically designed with extend piston areas being larger than retract piston areas, and are therefore called unbalanced actuators. Unbalanced actuator designs are lighter, shorter, and are less expensive than their balanced design counterparts. The area unbalance creates an excess or need for hydraulic fluid, depending on the direction of actuator motion. Unbalanced EHAs and IAPs require a bi-directional anti-cavitation valve and a reservoir to accommodate differential fluid flow in the actuator, and to prevent cavitation. This requirement results in a degradation of an EHA's actuator stiffness because one side of the piston is connected to the reservoir whenever a significant load is placed on the actuator. There exists, therefore, a significant need for a means of improving the stiffness capabilities of an EHA, such that it can duplicate the performance of a conventional central hydraulic system.
The present invention is a fast operating stiffness enhancement and load holding valve arrangement which provides energy reduction operating advantages for either conventional servovalve controlled actuators, IAPs and EHAs. Its maximun value is, however, in stiffness enhancement and energy reduction for EHAs and the description of the invention is directed relative to its use as part of an EHA.